Glucose fuels life. Most cells prefer glucose as a carbon and energy source; some cells require it. Cells have evolved numerous sophisticated mechanisms for sensing and responding to glucose. This is especially apparent in the yeast S. cerevisiae, in which several highly evolved glucose sensing mechanisms dictate the distinctive fermentative metabolism of yeast, a lifestyle it shares with many kinds of tumor cells. Our long-term goal is to understand how yeast cells sense and respond to glucose. Two glucose signal transduction pathways responsible for regulating gene expression are well known. One pathway works through the Snfl protein kinase and the Mig1 transcriptional repressor to cause glucose repression of gene expression. Another pathway that operates through the Snf3 and Rgt2 glucose sensors in the cell membrane which signal to the Rgt1 transcription factor in the nucleus to induce expression of genes encoding glucose transporters has more recently been revealed. Over the past several years, we have been able to trace the glucose signal uninterrupted from the glucose sensors at the cell surface to Rgtl in the nucleus, providing a model for this glucose signal transduction pathway. Over the next four years, the majority of our effort (about two-thirds) will be devoted to solidifying and revising this model (Aim 1), with the goal of contributing to establishment of this novel nutrient sensing pathway as one of the paradigms of signal transduction. We recently learned that these two glucose-signaling pathways are highly interconnected in a regulatory circuit that probably evolved to ensure the cells respond rapidly and sensitively to changing levels of glucose. We will attempt to learn the logic of this network by determining the effect of inactivating each of its arms (Aim 2). Finally, the manner by which S. cerevisiae (and many tumor cells) metabolizes glucose is so unusual that it begs explanation. We will devote the remainder of our effort to analysis of glucose signaling in S. kluyveri (Aim 3), a relative of S. cerevisiae that, because it of its conventional glucose metabolism, can be considered a model for the non-transformed, normal cell.